1. Field of the Invention
The present invention relates to a method and an apparatus for recording image information as an electrostatic latent image on an electrostatic recording material and for reading the electrostatic latent image having been recorded.
2. Description of the Related Art
In the field of radiography in medicine for example, radiation image information reading systems have been widely known. In radiography, in order to reduce a radiation dose on a patient and to improve diagnostic performance, a photoconductive material such as a selenium plate comprising a-Se (amorphous selenium) sensitive to X rays is used as an electrostatic recording material (a photosensitive material, a solid-state radiation detector). Radiation for recording, such as X rays, representing radiation image information is irradiated on the electrostatic recording material and a latent image charge representing the radiation image information is stored in a capacitor of the electrostatic recording material. Thereafter, reading light (an electromagnetic wave for reading) such as a laser beam scans the electrostatic recording material and a latent image represented by the latent image charge, that is, the radiation image information, is read by detecting a currrent generated by the scan within the electrostatic recording material by using plate electrodes or comb electrodes on both sides of the electrostatic recording material.
In such a system, the electrostatic recording material comprising the electrodes at both ends thereof and at least one photoconductive layer located inside of the electrodes are used. Radiation for recording is irradiated when a voltage for recording is supplied to the electrodes at both ends, and the latent image is formed in the capacitor of the electrostatic recording material. Thereafter, the electrodes at both ends are made to have the same potential (generally short-circuited), and the photoconductive layer of the electrostatic recording material is scanned with the light for reading, via an electrode having transmissivity to the reading light (hereinafter called an electrode on the reading light side). The latent image is electrically read by photoinductive discharge caused by pairs of electrons and holes (changed couples) generated at an interface between the electrode on the reading light side and the photoconductive layer. In this system, when the latent image is read, no electric current flows in a dark portion of the image while a larger current flows in a lighter portion of the image. This type of system in which the electrodes at both ends of the electrostatic recording material are short-circuited after recording and a larger current flows in a lighter portion of the image is called a positive system, and the electrostatic recording material used in the positive system is called a positive electrostatic recording material.
As a specific layer configuration of such a positive electrostatic recording material, several types can be used. For example, a configuration comprising a first conductive layer (electrode layer on the recording light side; hereinafter this layer is called a first conductive layer), a photoconductive layer for recording, a trap layer as a capacitor, a photoconductive layer for reading, and a second conductive layer (the electrode layer on the reading light side; hereinafter this layer is called a second conductive layer) can be used (U.S. Pat. No. 4,535,468, for example). Another configuration comprising a first conductive layer, a photoconductive layer for recording and reading, and a second conductive layer with a capacitor formed at an interface between the photoconductive layer and the second conductive layer can also be used (see Medical Physics, Vol. 16, No. 1, Jan/Feb 1989; P105-109). Alternatively, a layer configuration comprising a first conductive layer, an insulating layer, a photoconductive layer for recording and reading, and a second conductive layer, with a capacitor formed at an interface between the insulating layer and the photoconductive layer can be used.
The applicant of the present invention has also proposed a positive electrostatic recording material comprising a first conductive layer having transmissivity to radiation for recording, a photoconductive layer for recording exhibiting photoconductivity when receiving the radiation for recording, an electric charge transport layer acting approximately as an insulator to an electric charge having the same polarity as an electric charge in the first conductive layer while acting approximately as a conductor to an electric charge having the reverse polarity of the electric charge in the first conductive layer, a photoconductive layer for reading exhibiting photoconductivity when receiving reading light (an electromagnetic wave for reading), and a second conductive layer having transmissivity to the reading light, with these layers disposed in this order and a capacitor formed at the interface between the photoconductive layer for recording and the electric charge transport layer (Japanese Patent Applications Nos. 10(1998)-232824, 11(1999)-87922, and 11(1999)-89553).
However, in any of the positive electrostatic recording materials described above, a barrier electric field is created at the interface between the second conductive layer having transmissivity to the reading light and the photoconductive layer comprising a-Se or the like. As a result, a so-called photoelectromotive force noise problem, which is a problem caused by a current generated by the reading light even in an area exposed to 0mR recording radiation, occurs.
Furthermore, the photoelectromotive force noise will have local position dependency if the electrostatic recording material is continuously used. As a result, an artifact will be created.
In the photoconductive layer of the electrostatic recording material, a high-resistance amorphous material (having traps) such as a-Se is generally used. During the time from voltage supply (generally a high voltage) between the electrodes at both ends of the electrostatic recording material to the short circuit, an electric charge pours directly from the electrodes to the photoconductive layer. The charge is trapped as a space charge within the photoconductive layer or at the interfaces between the photoconductive layer and the electrodes, and leaked as a dark current in the photoconductive layer instead of being trapped as the space charge. This dark current is stored in the capacitor as a dark latent image upon reading and appears as a dark latent image noise in a reproduced image. This dark current is large in the beginning of the voltage supply, and reduced with time. Thereafter, the dark current reaches a certain leakage current value. In other words, the level of the dark current immediately after the voltage supply is larger than the level of the dark current in a stable state (the state of stable leakage current). This phenomenon is more conspicuous when the voltage is higher, and 10 minutes or more is necessary in some cases to reach the stable leakage current level. Furthermore, even if the stable state is temporarily established, the dark current level tends to reach the previous value when the voltage supply is resumed after a temporary voltage supply cut due to the short circuit between the electrodes at both ends. Therefore, the dark latent image due to the high-level dark current immediately after the voltage supply contributes largely to the reading noise. Moreover, the amount of the dark latent image changes with time from the voltage supply to irradiation of the recording light and with usage history. Therefore, correction of image data so as not to cause the dark latent image noise to appear in a reproduced image is difficult.
Moreover, as has been described above, the electric field generated by the space charge due to the supply of the recording voltage is created at the interfaces between the electrodes and the photoconductive layer. As a result of the short circuit prior to reading, a new electric field due to the history of the voltage supply (generally high voltage supply) and short circuit is created, and a problem of high-voltage supply history noise caused by irradiation of the light (reading light) in existence of the new electric field also occurs. Since this high-voltage supply history noise also changes with time and usage history, correction of this noise is also difficult as the dark latent image noise.
If the recording material is continuously used, the high-voltage supply history noise will have the local position dependency as the photoelectromotive force noise, which leads to generation of an artifact.
Meanwhile, the applicant has proposed an image quality degradation prevention method in Japanese Patent Application No. 10(1998)-232824. In this method, pre-exposure light is irradiated on the photoconductive layer for reading in a state where the voltage is being supplied but the radiation for recording has not been irradiated. By using rectification by the electric charge transport layer, the dark latent image and a residual image stored in the capacitor are reduced before the radiation for recording is irradiated.
Moreover, the applicant has also proposed reduction in the photoelectromotive force noise by adequately setting the barrier of the electric charge transport layer and the photoconductive layer for recording. In this method, a small amount of hole barrier is created and the holes are stored in the hole barrier by pre-exposure to make a flat band.
However, these methods are only applicable to the electrostatic recording material including the electric charge transport layer as described in Japanese Patent Application No. 10(1998)-232824 proposed by the applicant or the like, and the method described in Japanese Patent Application No. 10(1998)-232824 is not applicable to the other electrostatic recording materials described above.
Furthermore, hole barrier generation so as to cancel the photoelectromotive force at the interface between the electric charge transport layer and the photoconductive layer for recording is not easy.
Moreover, in the case where the dark current from the electrode on the reading light side is larger and a dark latent image having the reverse polarity of the electrostatic latent image is generated in the capacitor, the pre-exposure actually enhances the dark latent image.
The present invention has been conceived based on consideration of the above problems. An object of the present invention is therefore to provide an image information reading and recording method and an image information reading and recording apparatus enabling reduction in photoelectromotive force noise and stabilization of the noise in the case where a positive electrostatic recording material of optical reading type is used.
Another object of the present invention is to provide an image information reading and recording method and an image information reading and recording apparatus enabling reduction in a dark latent image formed immediately after voltage supply and stabilization of the dark latent image.
Still another object of the present invention is to provide an image information reading and recording method and an image information reading and recording apparatus enabling reduction and stabilization of high-voltage supply history noise generated by carrying out recording voltage supply and short circuit.
A first image information reading and recording method of the present invention is a method of carrying out reading and recording of image information by using an electrostatic recording material comprising a first electrode layer, a photoconductive layer exhibiting conductivity when receiving a recording electromagnetic wave representing the image information, and a second electrode layer disposed in this order, with a capacitor being formed between the first electrode layer and the second electrode layer for storing, as a latent image charge, an electric charge in accordance with the amount of energy of the electromagnetic wave. In the electrostatic recording material, the image information is recorded as an electrostatic latent image in the capacitor by exposure of the first electrode layer to the recording electromagnetic wave in a state where a recording voltage is applied between an electrode of the first electrode layer and an electrode of the second electrode layer, and the image information in accordance with the amount of the latent image charge is read in a state where the first electrode layer and the second electrode layer have the same potential.
The method comprises the steps of:
carrying out pre-reading in which pre-exposure light is irradiated on the photoconductive layer in a state where the electrode of the first electrode layer and the electrode of the second electrode layer have the same potential; and
carrying out the recording after the pre-reading is stopped.
The electrostatic recording material used in the present invention comprises a first electrode layer, a photoconductive layer, and a second electrode layer disposed in this order and a capacitor formed between the first electrode layer and the second electrode layer. The electrostatic recording material can be any positive electrostatic recording material of optical reading type. In order to form the capacitor, the electrostatic recording material may have other layers or minute conductive materials (microplates) disposed further, as described in U.S. Pat. No. 4,535,468 or Japanese Patent Application No. 10(1998)-232824, for example.
As the xe2x80x9crecording electromagnetic wave representing the image informationxe2x80x9d, transmissive radiation representing transmissive radiation image information obtained by irradiating radiation such as X rays on a subject, or light emitted by excitation caused by radiation, such as fluorescence representing the transmissive radiation image information obtained by irradiating the transmissive radiation on phosphor (a scintillator), or general visible light representing image information can be used.
The xe2x80x9cstate where the first electrode layer and the second electrode layer have the same potentialxe2x80x9d means not only the state where the electrodes of the two layers have the same potential by being connected directly, but also the state where the electrodes of the two layers actually have the same potential although a slight potential difference can exist between the two electrodes, such as in the case of using imaginary short circuit of an operation amplifier or using a resistor.
It is preferable for pre-exposure light for the pre-reading to have high intensity so that an effect such as light fatigue which will be explained later can be obtained efficiently. Meanwhile, it is not necessary for the pre-exposure light to be irradiated for a long time (such as 10 seconds, for example), and a comparatively short time (such as 1 millisecondxcx9c1 second) is sufficient. Therefore, in the first image information reading and recording method of the present invention, it is preferable for energy density of the pre-exposure light to be 100 Cd/m2 or more. More preferably, the energy density is approximately 1000 Cd/m2 or more and duration of the pre-exposure light irradiation is set not less than 1 millisecond and not greater than 1 second, preferably not less than 10 milliseconds and not greater than 100 milliseconds.
It is also preferable for the pre-exposure light to be irradiated immediately before the supply of the high voltage for recording (within 1 second, for example), when duration of the effect due to the pre-exposure light irradiation is taken into consideration.
Prior to the pre-reading, it is preferable for the first image information reading and recording method of the present invention to carry out pre-voltage supply in which a voltage having a predetermined magnitude and a predetermined polarity is applied for a predetermined amount of time between the electrode of the first electrode layer and the electrode of the second electrode layer.
xe2x80x9cPrior to the pre-readingxe2x80x9d means at least the case where the pre-voltage supply starts before the pre-reading, and the pre-voltage supply may be stopped slightly before or later than the start of the pre-reading.
It is more preferable for the first image information reading and recording method of the present invention to carry out, prior to the recording of the electrostatic latent image, pre-processing in which the pre-voltage supply and the subsequent pre-reading are repeated a predetermined number of times at each time the electrostatic latent image is recorded.
A duty ratio (Ton/T) of ON-period or OFF-period of the pre-voltage supply (Ton, Toff) to one cycle of the pre-processing (in time T) for the repeated pre-voltage supply and pre-reading is set to 50% or more, preferably approximately 90%.
If the pre-processing repeating the pre-voltage supply and the pre-reading at each recording of the electrostatic latent image is carried out as has been described above, the recording is not carried out immediately after the reading due to the time necessary for the pre-processing, and an operator has to halt photographing because of pre-processing. Especially, in the case where repetition of the pre-processing is time-consuming, the suspension of photographing becomes substantially long. Furthermore, consecutive photographing having a short time for each cycle becomes difficult to be carried out. Moreover, if consecutive photographing (regardless of the time for each photographing) is carried out, the amount of noise gradually changes in each pre-processing and does not have long-term stability. A second image information reading and recording method of the present invention is to solve such problems.
In other words, the second image information reading and recording method of the present invention is a method of carrying out reading and recording of image information by using an electrostatic recording material comprising a first electrode layer, a photoconductive layer exhibiting conductivity when receiving a recording electromagnetic wave representing the image information, and a second electrode layer, with these layers disposed in this order and a capacitor being formed between the first electrode layer and the second electrode layer for storing, as a latent image charge, an electric charge in accordance with the amount of energy of the electromagnetic wave. In the electrostatic recording material, the image information is recorded as an electrostatic latent image in the capacitor by irradiation of the recording electromagnetic wave on the first electrode layer in a state where a recording voltage is applied between an electrode of the first electrode layer and an electrode in the second electrode layer, and the image information in accordance with the amount of the latent image charge is read in a state where the first electrode layer and the second electrode layer have the same potential. The second image information reading and recording method comprises the steps of:
carrying out preparation processing repeating, a predetermined number of times, preparatory pre-voltage supply supplying a voltage having a predetermined magnitude and a predetermined polarity between the electrode of the first electrode layer and the electrode of the second electrode layer for a predetermined amount of time and preparatory pre-reading irradiating pre-exposure light on the photoconductive layer in a state where the electrode of the first electrode layer and the electrode of the second electrode layer have the same potential; and
carrying out the recording and the reading consecutively and a plurality of times after the preparation processing is stopped.
To carry out xe2x80x9cthe reading and the recording consecutively and a plurality of timesxe2x80x9d refers to so-called consecutive photographing. Upon carrying out the consecutive photographing after the preparation processing, not only the consecutive reading and recording may be carried out but also the consecutive reading and recording may be carried out in combination with the first method. For example, xe2x80x9cpre-reading+recording+readingxe2x80x9d may be defined as one cycle and be carried out consecutively. Alternatively, xe2x80x9cpre-voltage supply+pre-reading+recording+readingxe2x80x9d may be defined as one cycle and be carried out consecutively. Furthermore, xe2x80x9crepeated pre-voltage supply and pre-reading+recording+readingxe2x80x9d may be defined as one cycle and be carried out consecutively. These cycles may also be used in combination.
By carrying out the preparatory processing, time for the pre-processing immediately before the consecutive photographing can be reduced substantially, and can be cut completely in some cases.
In this second method, the energy density of the pre-exposure light for the preparatory pre-reading is set to 100 Cd/m2, preferably equal to or larger than 1000 Cd/m2. The duration of the pre-exposure light irradiation is set not less than 1 millisecond and not greater than 1 second, preferably not less than 10 milliseconds and not greater than 100 milliseconds.
The duration of the preparation processing for carrying out repeated preparatory pre-voltage supply and preparatory pre-reading is set at least 10 second or longer, preferably equal to or longer than 60 seconds. It is preferable for the duration to be set to almost equal to or more than the duration of the consecutive photographing. However, in order to sufficiently induce the effect of the preparation processing, the preparation processing is preferably carried out all the time the photographing is not carried out.
Furthermore, for the duration (Ton, Toff) of the ON-period or OFF-period of the preparatory pre-voltage supply in one cycle (the time T), 1xcx9c10 seconds is set for the ON-period, preferably approximately 1 second, and the duty ratio (Ton/T) is set to 70% or more, preferably approximately 90%.
It is preferable for the pre-exposure light or exposure light to be irradiated on the photoconductive layer from the side of the electrode layer on which the reading light is irradiated (usually from the second electrode layer side). It is also preferable for the pre-exposure light and the exposure light to have an almost constant amount of light over the entire surface of the electrostatic recording material.
A first image information reading and recording apparatus of the present invention is an apparatus for realizing the first image information reading and recording method. In other words, the first image information reading and recording apparatus of the present invention is an apparatus for carrying out reading and recording of image information by using an electrostatic recording material comprising a first electrode layer, a photoconductive layer exhibiting conductivity when receiving a recording electromagnetic wave representing image information, and a second electrode layer in this order, with a capacitor being formed between the first electrode layer and the second electrode layer for storing, as a latent image charge, an electric charge in accordance with the amount of energy of the electromagnetic wave. In the electrostatic recording material, the image information is recorded as an electrostatic latent image in the capacitor by irradiation of the recording electromagnetic wave on the first electrode layer in a state where a recording voltage is applied between an electrode of the first electrode layer and an electrode of the second electrode layer, and the image information in accordance with the amount of the latent image charge is read in a state where the first electrode layer and the second electrode layer have the same potential. The first image information reading and recording apparatus comprises:
voltage supply means for supplying a predetermined voltage between the electrode of the first electrode layer and the electrode of the second electrode layer;
pre-exposure means for irradiating pre-exposure light on the photoconductive layer; and
control means for controlling the pre-exposure means and the voltage supply means so as to carry out pre-reading by causing the pre-exposure light to be irradiated on the photoconductive layer in a state where the electrode of the first electrode layer and the electrode of the second electrode layer have the same potential and to record the electrostatic latent image after stopping the pre-reading, by causing the recording electromagnetic wave to be irradiated on the first electrode layer in a state where a voltage for recording is applied between the electrode of the first electrode layer and the electrode of the second electrode layer.
In the first image information reading and recording apparatus of the present invention, it is preferable for the pre-exposure means to set energy density of the pre-exposure light to be equal to or larger than 100 Cd/m2. It is also preferable for the pre-exposure means to set duration of the pre-exposure light irradiation to not less than 10 milliseconds and not greater than 1 second.
In the first image information reading and recording apparatus of the present invention, it is preferable for the control means to control the voltage supply means so that pre-voltage supply is carried out prior to the pre-reading. In the pre-voltage supply, a voltage having a predetermined magnitude and a predetermined polarity is applied between the electrode of the first electrode layer and the electrode of the second electrode layer for a predetermined period.
In the first image information reading and recording apparatus of the present invention, it is more preferable for the control means to control the pre-exposure means and the voltage supply means so as to carry out the pre-voltage supply and the subsequent pre-reading a predetermined number of times prior to each recording of electrostatic latent image.
A second image information reading and recording apparatus of the present invention is an apparatus for realizing the second image information reading and recoding method of the present invention. In other words, the second image information reading and recording apparatus is an apparatus for carrying out reading and recording of image information by using an electrostatic recording material comprising a first electrode layer, a photoconductive layer exhibiting conductivity when receiving a recording electromagnetic wave representing the image information and a second electrode layer disposed in this order, with a capacitor for storing, as a latent image charge, an electric charge in accordance with the amount of energy of the electromagnetic wave being formed between the first electrode layer and the second electrode layer. In the electrostatic recording material, the image information is recorded in the capacitor by exposure of the first electrode layer to the recording electromagnetic wave in a state where a recording voltage is applied between an electrode of the first electrode layer and an electrode of the second electrode layer, and the image information in accordance with the amount of the latent image charge is read in a state where the first electrode layer and the second electrode layer have the same potential. The second image information reading and recording apparatus comprises:
voltage supply means for supplying a predetermined voltage between the electrode of the first electrode layer and the electrode of the second electrode layer;
pre-exposure means for irradiating pre-exposure light on the photoconductive layer; and
control means for controlling the pre-exposure means and the voltage supply means so that preparation processing repeating a predetermined number of times preparatory pre-voltage supply supplying a voltage having a predetermined magnitude and a predetermined polarity between the electrode of the first electrode layer and the electrode of the second electrode layer and preparatory pre-reading irradiating the pre-exposure light on the photoconductive layer in a state where the electrode of the first electrode layer and the electrode of the second electrode layer have the same potential is carried out and the recording and the reading are carried out consecutively and a plurality of times after the preparation processing is stopped.
In the second image information reading and recording apparatus of the present invention, it is preferable for the pre-exposure means to set energy density of the pre-exposure light to be equal to or larger than 100 Cd/m2. It is also preferable for the pre-exposure means to set the duration of the pre-exposure light irradiation to not less than 1 millisecond and not greater than 1 second.
In any of the above methods and apparatuses, it is preferable for the electrostatic recording material to be of an optical reading type in which image information in accordance with the amount of the latent image charge is read by causing photoinductive discharge due to irradiation of the reading light (normally from the side of the second electrode layer) on the photoconductive layer exhibiting conductivity by exposure to the reading light in the state where the first electrode layer and the second electrode layer have the same potential.
The reading light is an electromagnetic wave for reading and not limited to visible light. The photoconductive layer exhibiting conductivity by receiving the reading light may also serve as the photoconductive layer exhibiting conductivity by receiving the electromagnetic wave for recording. Alternatively, the photoconductive layer exhibiting conductivity by receiving the reading light may be separate from the photoconductive layer exhibiting conductivity by receiving the electromagnetic wave for recording.
According to the first image information reading and recording method and apparatus of the present invention, the pre-reading in which the pre-exposure light is irradiated on the photoconductive layer is carried out in the state where the electrode of the first electrode layer and the electrode of the second electrode layer have the same potential, and an electrostatic latent image is recorded after the pre-reading is stopped. Therefore, a state of light fatigue (a state of trap accumulation) is temporarily created at the interface on which the pre-exposure light has been irradiated (an area of electron-hole pair formation), and photoelectromotive force noise generated upon irradiation of the reading light is reduced and stabilized due to the light fatigue state.
Furthermore, a space-charge state at the interfaces between the electrodes and the photoconductive layer can be alleviated and stabilized by the pre-exposure light. In other words, the high-voltage supply history noise can be reduced.
Moreover, as will be explained later, a current response showing a relationship between time t and a dark current I flowing in the electrostatic recording material upon the recording voltage supply and the short circuit is expressed as I xe2x88x9d=txe2x88x92n. In other words, when logI and logt are used, a state in which the current does not concentrate at a certain time constant is established. As behavior of the space charge at the interfaces between the electrodes and the photoconductive layer, a space charge having a short time constant (1 second or shorter) and having a long time constant (tens of millisecondsxcx9capproximately 1 minute or longer) exists. However, as has been described above, a comparatively short time (1 millisecondxcx9c1 second) is sufficient for the pre-voltage supply prior to the pre-reading, and only the space charge having the shorter time constant can respond if the pre-voltage supply is carried out for a short time and stopped.
Therefore, prior to the pre-reading, when the pre-voltage supply in which the voltage having the predetermined magnitude and the predetermined polarity is applied to the two electrodes for the predetermined amount of time is carried out, a space-charge state causing an apparent stable high-resistance state is created within the photoconductive layer or at the interfaces between the photoconductive layer and the electrodes, and a state of small dark latent image accumulation is also realized in the capacitor. For this reason, immediately after the recording voltage supply, conventional large dark latent-image noise is not generated but is in fact reduced and stabilized.
Moreover, as has been described above, since only the space charge having the short time constant can respond in the short voltage-supply. Therefore, if the pre-processing repeating the pre-voltage supply and the subsequent pre-reading the predetermined number of times is carried out prior to each recording of the electrostatic latent image, formation and release of shallow traps due to the space charge poured and discharged in a comparatively short time are facilitated and a state of stable accumulation of deep traps due to the space charge which is not poured or discharged unless a comparatively long time is spent can be established. This contributes to repeated stabilization of the high-voltage supply history noise caused by irradiation of light in existence of the electric field due to history of the voltage supply and the short circuit, and image data correction so as not to generate the high-voltage supply history noise in a reproduced image becomes possible.
Meanwhile, according to the second image information reading and recording method and apparatus of the present invention, the preparation processing repeating the preparatory pre-voltage supply and the preparatory pre-reading is carried out and the recording and the reading are carried out consecutively and a plurality of times after the preparation processing is stopped. Therefore, regardless of the time necessary for one cycle in the consecutive photographing, only the reading and the recording can be carried out repeatedly. In other words, the recording can be carried out immediately after the reading and the consecutive photographing can be carried out without long term pre-processing between photographing. Therefore, the second image information reading and recording method and apparatus can be excellent in long-term stability.